For your piece of mind Lizzy, can you go into your local VAT REG Garage, not the dealer or K.F?? ask them to pressure test the cooling system for leaks, this will tell them & you if the head Gasket has gone as said, have you run the car without water? should it be the head Gasket you are talking about a lot of money, but find out first, if it's the worst check around for prices and make sure you get this in writing before the job starts.